Sex Myths: STDs Are Transmitted Every Time You Have Sex

The Truth About STD Transmission

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There are a lot of sex myths that revolve around how to tell whether or not you have a cheating partner. One particularly poisonous idea is the myth that if your long-term partner suddenly has a new STD infection, it must mean they're cheating. After all, if you were the source of the STD, your partner would have been infected a long time ago. Right?

The basic assumption lurking behind this sex myth is that if you have an STD, it will be transmitted every time you have sex.

But that isn't actually true. In fact, there are a number of factors that can affect the likelihood of any particular STD being spread during any particular encounter where you are having sex.

Factors That Can Increase or Decrease the Likelihood of STD Transmission

What factors might affect whether or not you will pass on an infection during a sexual encounter?

  • the amount of virus/bacteria in your blood or in your genital secretions
  • whether you or your partner have any open sores or other wounds on your genitals, or in other areas where you'll have skin-to-skin contact during sex
  • the presence of other STDs in either you or your partner
  • the duration of the sexual encounter
  • whether or not safe sex is practiced
  • whether or not you employ lubricants, and what type of lubricants you use
  • the gender of the infected partner
  • the types of sex engaged in (e.g. anal, vaginal, oral)

The absolute risk of an STD being transmitted during any given sexual encounter is highly variable.

Just because your partner has only recently been diagnosed with an STD when the two of you have been together for a long time, it does not necessarily mean that they have been cheating on you.

In such a circumstance, there are actually several possible alternate explanations. These can be difficult to choose between, particularly if you weren't both screened for STDs prior to the beginning of the relationship.

For example, if you turn out to be asymptomatically infected with the same STD, there is a possibility that you were infected the whole time and only recently transmitted the disease to your partner. Alternatively, you both could have been infected and simply not recognized the symptoms until just now.

One Final Caveat

Just because STDs aren't necessarily transmitted every time you have sex doesn't mean you can assume they won't be transmitted during any particular time you are having sex. Your health isn't something you want to trust to luck, so it is your responsibility to do what you can to minimize your risk. Practicing safe sex, particularly when combined with appropriate testing, is a lot more reliable than relying on probability or prayer.

Further Reading

Top 10 Questions I Get From People Who Think They Might Have an STD. Learn how soon you can test for STDs after having had unprotected sex, whether there's any point to using a condom if you've already tested positive, and the answers to eight other common questions.

Myths About Safe Sex.  Common misconceptions about safe sex, and what you need to know in order to remain sexually healthy.

How To Protect Yourself From Sexually Transmitted Diseases. Everything you need to know about safe sex, from assessing your risk to protecting yourself, and much, much more.


Baeten, J.M. et al. (2011) "Genital HIV-1 RNA Predicts Risk of Heterosexual HIV-1 Transmission" Sci Transl Med 6 April 2011 3:77ra29

Boily M.C. et al. (2009) "Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies." Lancet Infect Dis. 2009 Feb;9(2):118-29.

Fleming D.T. et al. (2006)"Herpes virus type 2 infection and genital symptoms in primary care patients." Sex Transm Dis. 2006 Jul;33(7):416-21.

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